I grew up in Japan and my dad bought a great deal of teak furniture. When he
bought it the furniture salesman told him that the best way to protect his
furniture (several thousands worth in the 80's) was to use a high grade
motor oil. My dad's had that furniture for over 20 years,and it still looks
as dark as the day he bought it. They didn't have any synthetic motor oils
in the 80's, but I believe that's what my dad uses now. Good luck.